[0001] The present invention relates to a method of depositing phosphosilicate glass films
on the surfaces of substrates and partly formed semiconductor substrates such as silicon
substrates or aluminium wire patterns formed on silicon substrates.
[0002] As is known in the art, a phosphosilicate glass film is widely used as aprotective
coating, a
passi- vation film, an interlaminar insulating film of multiwire pattern layers, or
a solid diffusion source for an N-type impurity. To deposit phosphosilicate glass
films chemically, a so-called "atmospheric pressure chemical vapour deposition method"
is often used and this method results in the production of phosphosilicate glass films
having a uniform thickness and having a uniform phosphorous concentration. In the
atmospheric pressure method, a phosphosilicate glass film is deposited on a substrate
in, for example, a bell-jar type reaction apparatus from a reaction gas mixture including
monosilane (SiH4), phosphine (PH
3) and oxygen (0
2) under atmospheric pressure and at a temperature of between 350° and_450°C. However,
the atmospheric pressure chemical vapour deposition method has a disadvantage in that
it is not susceptible to mass production and a further disadvantage in that cracks
are likely to be generated in the deposited films during for example, a subsequent
annealing treatment at approximately 450°C for example to remove the surface state
formed in the semiconductor device afterthe deposition of the phosphosilicate glass
film. The generation of these cracks results in a decrease in the reliability of the
finished semiconductor device
[0003] In addition, a so-called "low pressure chemical vapour deposition method is also
used. In this method, a phosphosilicate glass film is deposited on a substrate from
a reaction gas mixture including a silicon compound gas and an oxidising gas under
a low pressure typically 1 Torr or less and at a temperature of between 350° and 450°C.
[0004] The deposition is carried out in a reaction tube made of, for example, quartz having
an inner diameter of 120 mm diameter and having a uniform heating zone of length of
approximately 100 mm. The gases are fed into one end of the tube and an exhaust outlet
is located at the other end. A number of substrates or partly formed semiconductor
devices are placed on a wafer holder with their principal surfaces parallel and substantially
perpendicular to the axis of the reaction tube.
[0005] However, there are probiems in these conventional low pressure chemical vapour deposition
methods because the thicknesses and the quality of the phosphosilicate glass film
deposited on the principal surfaces of the substrates varies with the amount of the
oxidising gas present. For instance, the variation in the film thickness of the phosphosilicate
glass films varies by a much as ± 30%, in this conventional apparatus. For this reason,
the phosphosilicate glass films are conventionally deposited at a mol ratio of 0
2/SiH
4 in the reaction feed gas of less than 2 (i.e. approximately 0.5 - 1.5). However, in
this case hydrogen remains and is included in the deposited phosphosilicate glass
films since the amount of oxidising gas present in the reaction gas mixture is insufficient
completely to oxidise the silicon compound gas. When the phosphosilicate glass film
contains a relatively large amount of hydrogen, cracks are likely to be generated
in the deposited phosphosilicate glass films. The formation of the cracks in the deposited
phosphosilicate glass films makes the phosphosilicate glass films useless and, also,
a relatively large amount of hydrogen causes an accumulation of phosphorus at the
interface of the substrate and the phosphosilicate glass film when the phosphorous
accumulated phosphosilicate glass films are heat-treated at a temperature of 900°C
or more.
[0006] Accordingly, an object of the present invention is to overcome these problems in
the deposition of phosphosilicate glass films and to provide a method of deposition
which enables stable phosphosilicate glass films having a uniform thickness and quality
and containing no substantial amount of hydrogen to be deposited on substrates under
mass production conditions.
[0007] In accordance with this invention a phosphosilicate glass film is deposited on a
number of substrates located in an elongate reaction tube with at least two gas feed
pipes having gas outlets distributed along them arranged along the inside of the reaction
tube with the silicon compound gas being introduced through one of the gas-feed pipes
and the oxidising gas being introduced through another of the gas feed pipes.
[0008] This arrangement enables a stable phosphosilicate glass film having a low phosphorus
concentration, and therefore, having an excellent water-vapour resistance to be deposited
on substrates using mass production techniques and enables stable phosphosilicate
glass films to be produced in which no substantial cracking occurs during a subsequent
annealing of the semiconductor device after the deposition of the phosphosilicate
glass film.
[0009] In accordance with a preferred embodiment of the present invention, a reaction gas
mixture having a mol ratio of phosphine (PH
3) to phosphine and monosilane (PH
3 + SiH
4) of 0.08 or less is used.
[0010] Particular embodiments of the method in accordance with this invention will now be
described and contrasted with the prior art with reference to the accompanying drawings;
in which:-
Figure 1 is a diagrammatic longitudinal section through a typical conventional reaction
tube for low pressure chemical vapour deposition;
Figure 2 is a diagrammatic cross-section through a typical conventional reaction tube;
Figure 3 is a diagrammatic longitudinal section of one embodiment of a reaction tube
for use in the present invention;
Figure 4 is a diagrammatic longitudinal section through another embodiment of a reaction
tube for use in the present invention;
Figure 5 is a diagrammatic cross-section of the reaction tubes shown in Figures 3
and 4;
Figure 6 is a graph illustrating the correlation between the deposition rate of the
phosphosilicate glass films and the position of the wafers along the reaction tube;
Figure 7 is a graph illustrating the correlation between the etch rate of the deposited
phosphosilicate glass films and the position of the wafers along the reaction tube;
and,
Figure 8 is a graph illustrating the correlation between the stress in the phosphosilicate
glass films deposited on wafers and the mol ratio of PH3 to PH3 + SiH4 in the deposition reaction gas mixture.
[0011] A conventional low pressure chemical vapour deposition method will now be described.
In this method, a phosphosilicate glass film is deposited on a substrate from a reaction
gas mixture including SiH
4, PH3 and 0
2 under a low pressure (e.g. 1 Torr or less) and at a temperature of 350 through 450°C.
[0012] For instance, as shown in Figures 1 and 2, a reaction tube 10 made of, for example,
quartz having an inner diameter of 120 mm diameter and a uniform heating zone length
of approximately 100 mm is used for this purpose. In the reaction tube 10, gas inlets
11 and 12 are attached for feeding a gas mixture of SiH
4 and PH
3 and 0
2 to one end thereof and an exhaust outlet 13 is attached to the other end thereof.
Around the reaction tube 10, a heater 14 is mounted. In the practice of the chemical
vapour deposition, plural substrates 15 for semiconductor devices made of, for example,
silicon (i.e. wafers) are placed on a wafer holder 16 in such a state that the main
surfaces of the wafers l5 are substantially vertically aligned to one another and
substantially perpendicularly intersect the central axis of the reaction tube 10.
Then, a gas mixture of SiH
4 and PH
3 and an 0
2.gas are fed from the gas inlets 11 and 12, while the inside of the reaction tube 10
is evacuated from the exhaust outlet 13 by means of a vacuum pump (not shown). Thus,
phosphosilicate glass films are formed on the main surfaces of the wafers 15 under
the conditions of, for example, a pressure of 0.5 - 1.0 Torr and a temperature of
approximately 425°C for 100 minutes.
[0013] Referring to Figures 3, 4 and 5, typical examples of reaction tubes used in the practice
of the present invention will now be explained. A reaction tube 20 or 30, made of,
for example, quartz having an inner diameter of 120 mm diameter and a constant heating
zone length of approximately 100 mm is provided with an 0
2 gas inlet 21 or 31 at one end thereof and an exhaust outlet 22 or 32 at the other
end thereof.
[0014] In the inside of the reaction tube 20 or 30, gas feed pipes 23 or 33 and 24 or 34
having plural small openings 25 or 35 pierced along the longitudinal direction thereof,
as typically shown in Figures 3, 4 and 5, are mounted in the substantially full length
of the constant heating portion of the reaction tube 20 or 30. In the practice of
the present invention, a gas mixture of PH
3 and SiH
4 is fed through the small openings 25 or 35 of the pipe 23 or 33, whereas an 0
2 gas is fed through the small openings 25 or 35 of the pipe 24 or 34. In the embodiment
of Figure 3, both gas feed pipes 23 and 24 are inserted into the reaction tube 20
from one end portion thereof. It should be noted that these gas feed pipes 23 and
24 can also be inserted into the reaction tube 20 from the opposite end portion of
the reaction tube 20. Contrary to this, in the embodiment of Figure 4, the gas feed
pipes 33 and 34 are inserted into the reaction tube 30 from the different end portions
thereof as shown in Fig. 4. It should be noted that the pipes 33 and 34 can also be
exchangeable with each other. As shown in Fig. 5, the small openings 25 or 35 are
desirably pierced in such a manner that the gas is outwardly and radially discharged
from the openings. A heater 26 or 36 is mounted around the reaction tube 20 or 30,
so that the constant heating zone of the reaction tube 20 or 30 can be uniformly heated
to a desired temperature.
[0015] Although the embodiments of the reaction tubes in which two gas feed pipes 23 and
24 or 33 and 34 are mounted are illustrated in Figs. 3, 4 and 5, it should be noted
that a gas mixture of PH 3 and SiH and/or an 0
2 gas can be fed through plural gas pipes and also that such plural pipes can be placed
at any configuration in the cross section of the pipes.
[0016] In the practice of the deposition of phosphosilicate glass film on substrates, e.g.
silicon wafers, plural wafers 27 or 37 are placed on a wafer holder 28 or 38 in such
a state that the main surfaces of the wafers 27 or 37 are substantially vertically
aligned to one another and substantially perpendicularly intersect the central axis
of the reaction tube 20 or 30. Then, a gas mixture of SiH
4 and PH
3 is fed from a gas inlet pipe 23 or 33 and an oxygen gas is fed from a gas inlet 21
or 31 and a gas feed pipe 24 or 34, while the inside of the reaction tube 20 or 30
is evacuated by means of a vacuum pump (not shown).
[0017] For instance, phosphosilicate glass films were deposited on the surfaces of silicon
wafers by using a reaction tube as shown in Figs. 4 and 5 as follows.
[0018] In the uniform heating zone of the reaction tube 30 having a length of about 1 m
and heated to a temperature of about 425°C, 70 pieces of sample silicon wafers 37
were placed at a distance of 15 mm. Further 20 pieces of dummy silicon wafers 37 were
placed at each end of said sample silicon wafers 37. Thus, 110 pieces of silicon wafers
37 were present in the uniform heating zone of the reaction tube 30. The system within
the reaction tube 30 was evacuated through the exhaust outlet 32 by means of a vacuum
pump (not shown) and the temperature of the uniform heating zone of the reaction tube
30 was heated to a temperature of approximately 425°C by means of the heater 36. Thus,
a reaction gas mixture of SiH
4 and PH
3 having a mol ratio of PH
3 to PH
3 and SiH
4 of 0.08 in a reaction gas mixture was fed through the pipe 33 at a gas feed rate
of 44 cc/min and an 02 gas was fed through the pipe 34 and the inlet nozzle 31 at
a gas feed rate of 30 cc/min and 90 cc/min, respectively, while the pressure of the
system is maintained at a vacuum pressure of 0.35 Torr. The gas feed volume ratio
of 0
2 to the mixture of SiH
4 was 3. The deposition of phosphosilicate glass films on the wafers 37 was carried
out for 50 minutes under the above-mentioned conditions. As a result of an infrared
absorption spectrophotometric analysis, no substantial amount of Si-H
2 (or Si
2O
3) bond was included in the deposited phosphosilicate glass films on the sample silicon
wafers.
[0019] On the other hand, the-above-mentioned deposition of phosphosilicate glass films
on silicon wafers was repeated, except that a conventional reaction tube 10 as shown
in Figs. 1 and 2 was used, instead of the reaction tube 37 as shown in Figs. 4 and
5. As a result of the infrared absorption spectrophotometric analysis of the resultant
phosphosilicate glass films deposited on the silicon wafers, it was observed that
a substantial amount of Si-H
2 (or Si
2O
3) bond was included in the resultant phosphosilicate glass films.
[0020] Furthermore, the correlations between the deposition ° rate (A/min) of the phosphosilicate
glass films on the silicon wafers and the position of the wafers placed in the heating
zone of the reaction tube are shown in Fig. 6. The position of the wafers is represented
by the distance (cm) from the center of the uniform heating zone of the reaction tube.
In Fig. 6, a curve A represents the correlation obtained from the above-mentioned
experiment according to the present invention in which the reaction tube 30 provided
with two gas feed pipes 33 and 34, as shown in Figs. 4 and 5 was used, whereas a curve
B represents the correlation obtained from the above-mentioned comparative experiment
in which the reaction tube 10 provided with no gas feed pipe therein was'used. As
is clear from the results shown in Fig. 6, according to the present invention, the
phosphosilicate glass films were substantially uniformly deposited on the surfaces
of the 70 pieces of silicon wafer and the uniformity of the deposited films was remarkably
improved as compared with the comparative experiment.
[0021] In addition, the correlations between the etch rate (A/min) of the phosphosilicate
glass films on the silicon wafers and the position of the wafers placed in the uniform
heating zone of the reaction tube are shown in Fig. 7. The etching was carried out
by using hydrogen fluoric acid under the conditions of a temperature of 20°C and the
ratio of EF : H
20 = 1.25 : 98.75. In Fig. 7, a curve A represents the correlation obtained from the
above-mentioned experiment according to the present invention in which the reaction
tube 30 provided with two gas feed pipes 33 and 34, as shown in Figs. 4 and 5 was
used, whereas a curve B represents the correlation obtained from the above-mentioned
comparative experiment in which the reaction tube 10 provided with no gas feed pipe
therein was used.
[0022] As is clear from the results shown in Fig. 7, according to the present invention,
the phosphosilicate glass films deposited on the surface of the 70 pieces of silicon
wafers were uniformly etched at a substantially uniform etch rate and the uniformity
in the etch rate of the deposited phosphosilicate glass films was remarkably improved
as compared with the comparative experiment. Since the etch rate of the phosphosilicate
glass films is proportional to the content of phosphorus in the deposited phosphosilicate
glass films, the above results of the present invention means that the phosphorus
was uniformly included in the phosphosilicate glass films deposited on the surface
of the 70 pieces of silicon wafers.
[0023] In addition to the above-mentioned experiments, the deposition of the phosphosilicate
glass films on silicon wafers was carried out by changing the gas feed rate. For instance,
in the case of the gas feed mol ratio of 0
2 to SiH
4 (0
2/SiH
4) of 2 (gas feed rate of SiH
4 and PH
3 was 44 cc/min), no Si-H
2 (or Si203) bond was included in the deposited phosphosilicate glass films. As the
above--mentioned ratio of 0
2 to SiH
4 is increased to 3 or 4, the- mass productivity of the deposition of the phosphosilicate
glass films on the surfaces of silicon wafers is gradually decreased. Therefore, the
ratio of 0
2 to SiH
4 and PH
3 is most desirably within the range of 2 through 3. Since the thickness of the phosphosilicate
glass films on the wafers depends upon the feed amount of SiH
4 and the like, the use of too small amount of the feed rate of SiH
4 is not preferable.
[0024] In the case where the deposition of the phosphosilicate glass films on the wafers
was carried out in a manner as described in the above-mentioned experiment, except
that the reaction tube 20 as shown in Fig. 3 was used, instead of the reaction tube
30 as shown in Fig. 4, results similar to the above-mentioned results were obtained.
[0025] Although oxygen was fed through the inlet 21 or 31 in the above-mentioned experiments,
nitrogen gas or the like can be fed through the inlet 21 or 31 to effect the smooth
gas flow of the reaction gas mixture in the reaction tube, as long as a sufficient
amount of oxygen can be fed through the pipe 24 or 34.
[0026] Thus, according to the present invention, the gas mixture of SiH
4 and PH
3 and the oxygen gas are separately fed to the reaction tube through the plural small
openings 25 or 35 pierced in the pipes 23 and 24 or 33 and 34 and oxygen gas can be
further fed from the gas inlet 21 or 31, no substantial amount of Si-H
2 (or Si203) bond is included in the deposited phosphosilicate glass films and the
phosphosilicate glass films having a uniform thickness and uniform film quality can
be deposited on substrates.
[0027] As mentioned hereinabove, according to the preferred embodiment of the present invention,
a reaction gas mixture having a mol ratio of PH
3 to PH
3 and SiH
4 (PH
3/PH
3 + S
iH4) of 0.08 or less, preferably 0.03 through 0.08, is used to obtain the deposited phosphosilicate
glass films having a low phosphorus content at a high mass productivity and in which
no substantial cracking occurs during, for example, the annealing step of the semiconductor
device.
[0028] The experimental results obtained from the use of the reaction tube 30 as shown in
Figs. 4 and 5 will now be explained.
[0029] In the uniform heating zone of the reaction tube 30 having a length of about 2 m
and heated to a temperature of about 425°C, sample silicon wafers 37 were placed with
a distance of 15 mm. The system within the reaction tube 30 was evacuated from the
exhaust outlet 32 by means of a vacuum pump (not shown) and maintained under a vacuum
pressure of 0.1 through 0.2 Torr during the deposition experiment. Thus, phosphosilicate
glass films were deposited on the surfaces of the silicon wafers in the form of discs
each having a diameter of 4 inches by using various gas feed rates of 0 gas and a
gas mixture of SiH
4 and PH
3. The oxygen gas was fed through the many small openings of the pipe 34, the gas inlet
31 and the gas mixture of SiH
4 and PH
3 was fed through the many small openings of the pipe 35.
[0030] The correlation between the stress of the phosphosilicate glass films deposited on
the wafers and the mol ratio of PH
3 (mol) to SiH
4 (mol) and PH
3 (mol) in the reaction gas mixture, which was obtained from a series of the experiments
is shown in Fig. 8. In Fig. 8, R
l represents a mol ratio of 0
2 (mol) to SiH
4 (mol) contained in the total reaction gas mixture. In the case of R
1 = 200, the deposition was carried out under an atmospheric pressure. The stress a
(x 10
8 dyne/cm
2) of each phosphosilicate glass film having a thickness of 1 micron at a room temperature
was determined according to a so-called Newton ring method.
[0031] As shown in Fig. 8, in the case of the mol ratio of PH
3 to PH
3 and SiH
4 of 0.08 or less, the deposited phosphosilicate glass films are under compressive
stress (i.e. the phosphosilicate glass films become convex). Contrary to this, in
the case of the mol ratio of PH
3 to PH
3 and SiH
4 of more than 0.08, the deposited phosphosilicate glass films are under tensile stress
(i.e. the phosphosilicate glass films become concave). This means that, in the case
where the phosphosilicate glass films are deposited on silicon wafers at a mol ratio
of PH
3/PH
3 + SiB
4 of 0.08 or less, it is difficult for cracking to be generated in the deposited phosphosilicate
glass films since the resultant deposited films are under compressive stress. This
is because, in the case where the deposited phosphosilicate glass films are under
compressive stress, the stress does not become larger at a room temperature when the
films are annealed at a room temperature.
[0032] Referring to the dotted curve in Fig. 8, in the case where the phosphosilicate glass
films are deposited at R
1 of 200 according to a so-called atmospheric pressure chemical vapor deposition method,
the cracking readily occurs especially in the field of low phosphorus content (i.e.
the tensile stress is increased with the decrease in PH
3 content in the reaction gas mixture).
[0033] In order to actually observe the generation of the cracking in the deposited phosphosilicate
glass films, the following two series of experiments were carried out.
[0034] In the first experiments, aluminum layers each having a thickness of 1 micron were
first deposited on the entire main surfaces of silicon wafers in the form of discs
each having a diameter of 4 inches and, then, phosphosilicate glass films each having
a thickness of 1 micron were deposited on the surfaces of the aluminum layers. The
deposition of the phosphosilicate glass films was carried out under the following
conditions.
Pressure: 0.1 - 0.2 Torr
Temperature: 425°C
Mol Ratio of O2/SiH4 (R1): listed in Table 1 below
Mol Ratio of PH3/PH3+SiH4 (R2): listed in Table 1 below
[0035] The generation of cracking of the resultant phosphosilicate glass films was observed
at a temperature of 450°C. The results are shown in the following Table 1.

[0036] In the second experiment, a specified aluminum pattern having a thickness of 1 micron
was formed on the main surfaces of silicon wafers and, then, phosphosilicate glass
films each having a thickenss of 1 micron were deposited on the aluminum pattern.
The deposition of the phosphosilicate glass films was carried out under the following
conditions.
Pressure: 0.1 - 0.2 Torr
Temperature: 425°C
R1: listed in Table 2 below
R2: listed in Table 2 below
[0037] The generation of cracking of the resultant phosphosilicate glass films was observed
at a temperature of 450°C. The results are shown in the following Table 2.

[0038] In the results shown in Tables 1 and 2, lc, 2c, 3c, 4c and 5c represent the number
of the heat cycles at 450°C in nitrogen atmosphere (1 cycle was 30 minutes).
[0039] As is clear from the results shown in Tables 1 and 2, in the phosphosilicate glass
films deposited at a mol ratio of PH
3 to PH
3 and SiH
4 (i.e. R
2) of 0.08 or less, no cracking was generated in the (practical) area of the semiconductor
device irrespective of the mol ratio of 0
2 to SiH
4 (i.e. R
1). It should be noted that similar results would be obtained in the phosphosilicate
glass films as interlaminar insulating films and passivation films.
[0040] As is clear from the results shown in Tables 1 and 2, no cracking was also generated
in the phosphosilicate glass films deposited at a mol ratio R
2 of 0.12 to 0.20. However, in the case where the phosphorus content in the phosphosilicate
glass film becomes large, the water vapor resistance is undesirably decreased because
of the absorption of water due to the presence of phosphoric acid formed in the film.
Especially when the phosphosilicate glass films are used as a cover film for semiconductor
device, the decrease in the-water vapor resistance of the film should be avoided.
For this reason, the use of R
2 of 0.08 or less is desirable. On the other hand, when the phosphosilicate glass film
is used as an interlaminar insulation film of wire pattern layers, the films deposited
at a mol ratio R
2 of larger than 0.08 are acceptable and rather preferable. This is because, when the
heat treatment is carried out to make the edge portion of the through-hole gentle,
more gentle edge portions can be readily formed due to the fact that the phosphosilicate
glass film is easy to melt as the phosphorus content of the film is high.
[0041] As mentioned hereinabove, according to the present invention, since the deposition
of the phosphosilicate glass films is carried out according to a low pressure method,
the phosphosilicate glass films can be simultaneously deposited on the surfaces of
a lot of substrates to be treated placed in the reaction tube at an extremely high
mass productivity.
[0042] Furthermore, since a deposition gas mixture having an extremely low mol concentration
of PH
3 is used in the deposition of phosphosilicate glass films of the present invention,
the phosphorus content in the resultant phosphosilicate glass films is extremely low
and, as a result, the water-vapor resistance of the deposited films is excellent.
In addition, since the phosphosilicate glass films having compressive stress are formed
according to the present invention, the generation of heat cracking in the phosphosilicate
glass films is largely obviated.
[0043] Consequently, according to the present invention, since the phosphosilicate glass
films having the above--mentioned advantages can be deposited, as interlaminar insulating
films or passivation films, on the substrates for semiconductor devices suitable for
use in semiconductor integrated circuits, the reliability of the semiconductor devices
is remarkably improved.